Concurrent sexual partnerships, where individuals have two or more sexual partners at the same time, are major contributors to the transmission of sexually transmitted infections (STIs). Yet, very little is known about the prevalence and predictors of sexual concurrency in populations of men who have sex with men. This presentation reports on the findings of a national community-based survey of 1,049 Australian gay men aged 18–39 years, who were asked about their sexual partners over a 12-month period. Of those who reported having sex in the past 12 months (N = 901), 28% reported at least one period in which they had concurrent sexual partners. Of this group, 57% had sex concurrently with two partners, 25% with three partners, and 18% with four or more partners. Worryingly, 64% reported having unprotected sex with one or more of their concurrent partners. A multivariate logistic regression found sexual concurrency was just as likely among men of all ages. However, sexual concurrency was significantly more prevalent among those on higher incomes (P = 0.01), who were not in an ongoing relationship (P = 0.009), and who reported having large numbers of sexual partners in the past 12 months (P < 0.001). Of men who were tested for an STI in the past 12 months, STIs were more prevalent among those who reported sexual concurrency (20% vs. 13%, P = 0.04). In all, concurrent sexual partnerships appear to be common among 18–39 year old gay men in Australia. Discussion will focus on ways in which these data provide information for understanding and controlling STI epidemics in populations of gay men and other men who have sex with men.